1. Technical Field
The present disclosure relates to a microelectromechanical structure, in particular a uniaxial or biaxial gyroscope, provided with an increased sensitivity to detection of angular velocities, in particular angular velocities of pitch and roll.
2. Description of the Related Art
As is known, micromachining techniques enable manufacturing of microelectromechanical structures or systems (MEMS) within layers of semiconductor material, which have been deposited (for example, a polycrystalline-silicon layer) or grown (for example, an epitaxial layer) on sacrificial layers, which are removed via chemical etching. Inertial sensors, accelerometers, and gyroscopes provided with this technology are encountering an increasing success, for example, in the automotive field, in inertial navigation, or in the sector of portable devices.
In particular, integrated gyroscopes are known, which are made of semiconductor material and obtained with MEMS technology. These gyroscopes operate on the basis of the theorem of relative accelerations, exploiting the Coriolis acceleration. When an angular velocity is applied to a mobile mass moving at a linear velocity, the mobile mass “feels” an apparent force, referred to as Coriolis force, which determines a displacement thereof in a direction perpendicular to the direction of the linear velocity and to the axis about which the angular velocity is applied. The mobile mass is supported via springs that enable its displacement in the direction of the apparent force. Based on the Hooke's law the displacement is proportional to the apparent force, so that the Coriolis force, and the value of the angular velocity that has generated it, can be determined from the displacement of the mobile mass. The displacement of the mobile mass can, for example, be detected in a capacitive way, determining, in resonance conditions, capacitance variations caused by the movement of mobile electrodes, fixed with respect to the mobile mass and combfingered with fixed electrodes.
The published US patent applications US 2007-0214883, US 2009-0064780, and US 2009-0100930, filed by the applicant of the present application, disclose an integrated microelectromechanical sensor with a rotary driving movement and sensitive to angular velocities of pitch, roll, and yaw about respective axes of detection.
This microelectromechanical sensor comprises a single driving mass, anchored to a substrate in a single central point, and actuated with rotary motion about an axis passing through the central point and orthogonal to the plane of the driving mass. The rotation movement of the driving mass makes it possible to obtain in the plane of the mass two components of driving velocity, orthogonal with respect to one another. Through openings are provided within the driving mass, and corresponding sensing masses are arranged within the through openings; the sensing masses are enclosed in the overall dimensions of the driving mass, are suspended with respect to the substrate, and are connected to the driving mass via flexible elements. Each sensing mass is fixed with respect to the driving mass during its rotary motion, and moreover has a further degree of freedom of movement as a function of an external stress, in particular a Coriolis force, acting on the sensor. The flexible elements, thanks to their particular construction, enable the sensing masses to perform a rotary movement of detection about an axis belonging to the plane of the sensor, or alternatively a linear movement of detection along an axis belonging to the plane of the sensor, respectively, in response to a Coriolis acceleration acting in a direction perpendicular to the plane and to a Coriolis acceleration acting in a direction belonging to the plane. The detection movement is in any case substantially decoupled from the actuation movement of the driving mass. This microelectromechanical structure, in addition to being compact (in so far as it envisages a single driving mass enclosing in its overall dimensions a plurality of sensing masses), makes it possible to obtain with minor structural modifications, a uniaxial, a biaxial, or a triaxial gyroscope (and/or possibly an accelerometer, according to the electrical connections implemented), at the same time ensuring an excellent decoupling of the driving dynamics from the detection dynamics.
FIG. 1 shows an exemplary embodiment of a biaxial microelectromechanical gyroscope, designated by 1, according to the teachings disclosed in the aforesaid patent applications.
The gyroscope 1 is provided in a die 2 comprising a substrate 2a made of semiconductor material (for example, silicon), and a frame 2b; the frame 2b defines inside it an open region 2c, overlying the substrate 2a, and designed to house detection structures of the gyroscope 1 (as described in detail hereinafter). The open region 2c has a generally square or rectangular configuration in a horizontal plane (in what follows, plane of the sensor xy), defined by a first horizontal axis x and a second horizontal axis y, fixed with respect to the die 2. The frame 2b has sides substantially parallel to the horizontal axes x, y. Contact pads 2d (so-called “die pads”) are arranged along a side of the frame 2b, aligned, for example, along the first horizontal axis x. In a way not illustrated, the die pads 2d make it possible to electrically contact from the outside the detection structures of the gyroscope 1. The die pads 2d have an axis of symmetry, in this case coinciding with the second horizontal axis y (orthogonal to their direction of alignment), being arranged in equal number and in a specular way on opposite sides of the second horizontal axis y.
In particular, the first and second horizontal axes x, y correspond to a first axis of detection and a second axis of detection of the gyroscope 1 (more precisely, to a pitch axis and to a roll axis), about which corresponding angular velocities {right arrow over (Ω)}x (pitch angular velocity) and {right arrow over (Ω)}y (roll angular velocity) are detected.
In detail, the gyroscope 1 comprises a driving structure, housed within the open region 2c and formed by a driving mass 3 and by a driving assembly 4.
The driving mass 3 has a generally circular geometry with radial symmetry, having a substantially planar configuration with main extension in the plane of the sensor xy, and negligible dimension, with respect to the main extension, in a direction parallel to a vertical axis z, forming with the first and second horizontal axes x, y a set of three orthogonal axes, fixed with respect to the die 2. For example, the driving mass 3 has in the plane of the sensor xy basically the shape of an annulus, and defines at the center an empty space 6, the center O of which coincides with the center of gravity and the center of symmetry of the entire structure.
The driving mass 3 is anchored to the substrate 2a by means of a first anchorage 7a placed at the center O, to which it is connected through first elastic anchorage elements 8a. In the example, the first elastic anchorage elements 8a extend, forming a cross, from the center O parallel to the first and second horizontal axes x, y. The driving mass 3 is anchored to the substrate 2a by means of further anchorages 7b, set on the outside of the same driving mass 3, to which it is connected by means of further elastic anchorage elements 8b. For example, the further elastic anchorage elements 8b are of the folded type, are four in number, and are aligned in pairs along the first and second horizontal axes x, y, in such a way that the further anchorages 7b are set, in pairs, on opposite sides of the driving mass 3 with respect to the empty space 6, at the vertex of a cross centered in the center O. The first and further elastic anchorage elements 8a, 8b enable a rotary movement of the driving mass 3 about a driving axis passing through the center O, parallel to the vertical axis z and perpendicular to the plane of the sensor xy.
The driving mass 3 has: a first pair of through openings 9a, 9b, aligned in a diametric direction along the first horizontal axis x (pitch axis), and set on opposite sides with respect to the empty space 6; and a second pair of through openings 9c, 9d, aligned in a diametric direction along the second horizontal axis y (roll axis), and set on opposite sides with respect to the empty space 6. In particular, each through opening 9a-9d has in the plane of the sensor xy the shape of a radial annulus sector, having arc-shaped internal and external sides and radially extending lateral sides. In addition, the through openings 9a, 9b of the first pair are symmetrical with respect to the second horizontal axis y, and the through openings 9c, 9d of the second pair are symmetrical with respect to the first horizontal axis x.
The driving assembly 4 comprises a plurality of driven arms 10, extending externally from the driving mass 3 in a radial direction and in such a way that they are set at equal angular distances apart from one another, and a plurality of first and second driving arms 12a, 12b, extending parallel to, and on opposite sides of, respective driven arms 10. Each driven arm 10 carries a plurality of first electrodes 13, extending perpendicular to, and on both sides of, the same driven arm. In addition, each of the first and second driving arms 12a, 12b carries respective second electrodes 14a, 14b, which extend towards the respective driven arm 10, and are combfingered with the corresponding first electrodes 13. The first driving arms 12a are set all on one and the same side of the respective driven arms 10, and are biased all at one and the same first potential. Likewise, the second driving arms 12b are set all on the opposite side of the respective driven arms 10, and are biased all at one and the same second potential. A driving circuit (not illustrated) is connected to the second electrodes 14a, 14b for applying the first and second potentials and determining, by means of the mutual and alternating attraction of the electrodes, an oscillatory rotary motion of the driving mass 3 about the driving axis, at a given frequency of oscillation.
The gyroscope 1 further comprises a first pair of acceleration sensors with axis parallel to the vertical axis z, and in particular a first pair of sensing masses 16a, 16b, set within a respective through opening 9a, 9b so as to be completely enclosed and contained in the overall dimensions of the driving mass 3 in the plane of the sensor xy. Each of the sensing masses 16a, 16b has a shape corresponding to that of the respective through opening, and consequently has, in plan view, the general shape of a radial annulus sector. In detail, each of the sensing masses 16a, 16b has a first portion 17, which is wider, and a second portion 18, which is narrower (along the first horizontal axis x) but has in any case dimensions comparable with those of the first portion 17, the two portions being connected by a connecting portion 19, which is shorter (in a direction parallel to the second horizontal axis y) than the first and second portions 17, 18, and consequently has a center of gravity G located within the corresponding first portion 17. In greater detail, the first portion 17 has an outer side that is arc-shaped and concave and lateral sides extending radially, and the second portion 18 has an outer side that is arc-shaped and convex and lateral sides extending radially, aligned along the lateral sides of the first portion 17. Each of the sensing masses 16a, 16b is supported by a pair of elastic supporting elements 20 extending from the connecting portion 19 to the driving mass 3, connecting thereto, parallel to the second horizontal axis y (the elastic supporting elements 20 extending consequently in an intermediate position with respect to the width of the respective sensing mass). The elastic supporting elements 20 extend within recesses 21 provided on opposite sides of the corresponding sensing mass 16a, 16b, at a distance be from its center of gravity G. They form torsional springs rigid in regard to the rotary motion of the driving mass 3 (so that the sensing masses 16a, 16b follow the driving mass 3 in its driving motion), and also enable rotation of the sensing masses about an axis of rotation parallel to the second horizontal axis y and belonging to the plane of the sensor xy, and hence their movement out of the plane of the sensor xy (a movement that is not instead allowed to the driving mass 3).
The gyroscope 1 further comprises a second pair of acceleration sensors with axis parallel to the vertical axis z, and in particular a second pair of sensing masses 16c, 16d, housed within the through openings 9c, 9d, and completely enclosed and contained by the driving mass 3. The sensing masses 16c, 16d are obtained from the rotation through 90° of the sensing masses 16a, 16b with respect to the center O, and consequently the corresponding elastic supporting elements 20 extend parallel to the first horizontal axis x and enable rotation out of the plane of the sensor xy, about an axis of rotation parallel to the first horizontal axis x.
A pair of first and second sensing electrodes 22, 23 is set underneath the first and second portions 17, 18 of each of the sensing masses 16a-16d. The first and second sensing electrodes 22, 23 are polycrystalline-silicon regions formed on top of the substrate 2a and having a substantially trapezoidal shape and dimensions substantially the same as one another and corresponding to those of the second portion 18. The first and second sensing electrodes 22, 23 are separated, respectively, from the first and second portions 17, 18, by an air gap, and form, together with the first and second portions 17, 18 respective sensing capacitors. The first and second sensing electrodes 22, 23 of each sensing mass 16a-16d are connected in a differential way to a read circuit of the gyroscope 1 (not illustrated) via the connection pads 2d. 
In use, the gyroscope 1 is able to operate as a biaxial gyroscope, and to detect a (pitch) angular velocity {right arrow over (Ω)}x about the first horizontal axis x, and a (roll) angular velocity {right arrow over (Ω)}y about the second horizontal axis y.
With reference also to FIG. 2, the rotary movement of the driving mass 3 and of the sensing masses 16a-16d about the driving axis can be represented by a driving-velocity vector {right arrow over (v)}a, tangential to the circumference that describes the path thereof. In particular, the rotary motion about the first horizontal axis x or the second horizontal axis y with angular velocity {right arrow over (Ω)}X, {right arrow over (Ω)}y causes a Coriolis force (designated by {right arrow over (F)}C) acting on the entire structure, proportional to the vector product between the angular velocity {right arrow over (Ω)}X, {right arrow over (Ω)}y and the driving velocity {right arrow over (v)}a, and hence directed along the vertical axis z. On the entire structure, considered as a single rigid body, it is hence possible to identify a distribution of Coriolis forces, the value of which increases as the distance from the center O increases. The resultants of the Coriolis force {right arrow over (F)}C acting on the sensing masses 16a-16d at the corresponding center of gravity G, cause rotation of the same sensing masses, which move out of the plane of the sensor xy, about an axis parallel to the first horizontal axis x or the second horizontal axis y and passing through the first elastic supporting elements 20. This movement is enabled by the torsion of the first elastic supporting elements 20. Instead, the configuration of the first and further elastic anchorage elements 8a, 8b is such as to inhibit, to a good approximation, the movement of the driving mass 3 out of the plane of the sensor xy, in this way enabling the effective decoupling of the motion of detection of the sensing masses with respect to the driving motion. Displacement of the sensing masses 16a-16d out of the plane of the sensor xy causes the approach/moving away of the first portion 17 to/from the respective sensing electrode 22, and a corresponding moving away/approach of the second portion 18 from/to the respective sensing electrode 23. There follows a differential capacitive variation of the sensing capacitors associated to one and the same sensing mass and to sensing masses of one and the same pair, the value of which is proportional to the angular velocity {right arrow over (Ω)}x, {right arrow over (Ω)}y which can hence be determined via a suitable read circuit operating according to a differential scheme.
In particular, given that the reading scheme is differential, the presence of a pair of electrodes 22, 23 for each of the sensing masses 16a-16d enables automatic rejection of linear spurious accelerations along the vertical axis z. In addition, a rotation about the first horizontal axis x is not felt by the second pair of sensing masses 16c, 16d, in so far as the resultant Coriolis force {right arrow over (F)}C is zero (since the vector product between the angular velocity {right arrow over (Ω)}x and the corresponding driving velocity {right arrow over (v)}a goes to zero). Likewise, the rotation about the second horizontal axis y is not felt for similar reasons by the first pair of sensing masses 16a, 16b, so that the two axes of detection are not affected and are substantially decoupled.
The particular conformation of the sensing masses 16a-16d enables increase in the sensitivity of the gyroscope 1 (as compared to the use of other geometries for the same first sensing masses). In particular, the corresponding center of gravity G is positioned at a distance bC from the elastic supporting elements 20 (and from the corresponding axis of rotation out of the plane of the sensor xy) that is greater than that of the center of gravity of any rectangular mass that can be inscribed in one and the same sector of the driving mass 3 and is supported by elastic supporting elements extending along the same axis of rotation. Consequently, it is possible to obtain a higher torque and hence a greater movement of rotation out of the plane of the sensor xy, and in this way to obtain a higher sensor sensitivity.
In addition, the presence of the further elastic anchorage elements 8b, located outside the driving mass 3, enables increase of the stiffness of the driving mass 3 in regard to the movements out of the plane of the sensor xy, and hence increase of the decoupling between the driving movement and the detection movements.
Even though the gyroscope described represents a considerable improvement over known gyroscopes, it is not optimized from the standpoint of manufacturing simplicity and efficiency in terms of the corresponding electrical characteristics.